Painting rims uncle

My buddy painted his boat with a roller. It looked like it was sprayed. He bought Interlux flow additive to put in the paint. He said it went on looking rough but after a few minutes it flows out. No idea if stuff like that is available for auto paint. Maybe hot weather reducer or this?
https://www.rustoleum.com/product-c...ives/paintbooster-flow-control-paint-additive